Electrophoretic displays (EPDs) are known non-emissive displays suitable for use in the display of digital, alphanumeric, analog or graphical information. An EPD system typically comprises a suspension of pigment particles in a dielectric liquid held between two electrodes, at least one of which is transparent. Applied potential across the electrodes causes the charged particles to migrate to one or the other electrode. Where the suspension includes pigment particles and a dielectric liquid of contrasting colors, the movement of the pigment particles will cause images to be displayed, which are visible through the transparent electrode(s) or a viewing sheet.
EPDs have attributes of excellent brightness and contrast, wide view angles, bistability and low power consumption when compared with liquid crystal displays. The basic technology associated with the development and manufacture of EPD's has been disclosed in U.S. Pat. Nos. 5,961,804, 4,732,830, 4,680,103, 4,298,448, 4,732,830 and cited patents therein; and the disclosures of these patents are incorporated herein by reference in their entirety.
Among the advantages of an EPD over other types of flat panel displays is the very low power consumption. This salient advantage makes the EPD particularly suitable for portable and battery powered devices such as laptops, cell phones, personal digital assistants, portable electronic medical and diagnostic devices, global positioning system devices and the like.
In order to prevent undesired movements of particles such as sedimentation in the dielectric fluid, partitions were proposed between the two electrodes for dividing the space into smaller cells. See for example, M. A Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8, pp. 1148-1152 (1979). However, the filling and sealing processes of electrophoretic fluids in these cells are of very low throughput and high cost.
Attempts have been made to enclose the suspension in microcapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describe microencapsulated EPDs. These displays have a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of charged pigment particles suspended in a dielectric fluid that visually contrasts with the particles.
U.S. Pat. Nos. 6,930,818 and 6,933,098 (the '818 and '098 Patents), assigned to the same inventive entity, describe EPDs comprising microcups of well-defined shape, size and aspect ratio, filled with charged pigment particles dispersed in an optically contrasting dielectric solvent. The methods of manufacturing such microcups have also been described in the '818 and '098 Patents and are incorporated by reference herein in their entirety.
The methods of preparing the improved color EPD disclosed in the '818 and '098 Patents include laminating preformed empty microcups with a layer of positively working photoresist, imagewise exposure of the positive photoresist to selectively open a certain number of microcups, developing the photoresist, filling the selectively opened microcups with a colored electrophoretic fluid, and sealing the filled microcups. The sequence of opening and developing the photoresist, and filling and sealing of the microcups may be performed in iterative steps to create sealed microcups filled with electrophoretic fluids of different colors.
One aspect of the above cited '818 Patent relates to a novel roll-to-roll process and apparatus which permit the manufacture of a display to be carried out continuously by a synchronized photolithographic process. The synchronized roll-to-roll process and apparatus are also useful for manufacturing liquid crystal displays (LCD) and other structures and assemblies for electronic devices.
The '098 Patent also describes the manufacture of a plurality of microcups which are formed integrally with one another as portions of a structured two-dimensional array assembly, being formed upon a support web including a patterned conductor film, such as addressable indium-tin oxide (ITO) lines. Each microcup of the array assembly is filled with a suspension or dispersion of charged pigment particles in a dielectric solvent, and sealed to form an electrophoretic cell.
The substrate web upon which the microcups are formed includes a display addressing array comprising pre-formed conductor film, such as ITO conductor lines. The conductor film (ITO lines) and support web are coated with a radiation curable layer. The film and radiation curable layer are then exposed imagewise to radiation to form the microcup wall structure. Following exposure, the unexposed area is removed by using a developer, leaving the cured microcup walls bonded to the conductor film/support web. The imagewise exposure may be by UV or other forms of radiation through a photomask to produce an image or predetermined pattern of exposure of the radiation curable material coated on the conductor film. Although it is generally not required for most applications, the mask may be positioned and aligned with respect to the conductor film, i.e., ITO lines, so that the transparent mask portions align with the spaces between ITO lines, and the opaque mask portions align with the ITO material. The '818 and '098 Patents also describe a microcup array prepared by a process including embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold. The precursor layer may be hardened by radiation, cooling, solvent evaporation, or other means. Solvent-resistant and thermomechanically stable microcups having a wide range of size, shape, pattern, and opening ratio can be prepared by either one of the aforesaid methods.
In addition, the above cited '818 and '098 Patents describe the manufacture of a monochrome EPD from a microcup assembly by filling the microcups with a single pigment suspension composition, sealing the microcups, and finally laminating the sealed array of microcups with a second conductor film pre-coated with an adhesive layer.
The '818 and '098 Patents also relate to the manufacture of a color EPD from a microcup assembly by a process of sequential selective opening and filling of predetermined microcup subsets. The process includes laminating or coating the pre-formed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the photoresist, filling the opened microcups with a colored electrophoretic fluid, and sealing the filled microcups by a sealing process. These steps may be repeated to create sealed microcups filled with electrophoretic fluids of different colors for a color EPD.
The '098 Patent also describes a synchronized roll-to-roll photolithographic exposure method and apparatus, which may be employed for a number of useful processes, including the process of making the microcup array and the process of selectively filling of the array of microcups to form a color display assembly. The imagewise roll-to-roll photolithographic exposure is performed through a moving photomask synchronized with a moving web substrate, to permit imagewise exposure of the workpiece (e.g., microcup array or color display) in a continuous or semi-continuous and seamless manner. This synchronized roll-to-roll exposure photolithographic process is also amenable for the manufacture of color displays.
The synchronized roll-to-roll process may be adapted to the preparation of a wide range of structures or discrete patterns for electronic devices formable upon a support web substrate, e.g., patterned ITO films, flexible circuit boards or the like.
However, where the manufacture of high contrast ratio color displays requires the use of microcups that are structurally wide and deep, resulting in relatively thin partitions between the microcups, defects in the finished displays are often observed.
These defects may be attributed to, in part, defects in various individual manufacturing steps, including poor mechanical and dissolution characteristics of the positive photoresist, error in the undercutting of the photoresist by a developer, and poor adhesion of the photoresist to the walls or partitions of the microcups. These manufacturing defects appear as indefinite microcup structures and adversely affect the quality of the display images. Therefore, there exists a need for a defect free manufacturing process for imagewise opening the microcups and filling them with selectively colored electrophoretic fluid for color EPDs.